Acharya Shree Ram, Turkowski Volodymyr, Zhang G P, Rahman Talat S
Department of Physics, University of Central Florida, Orlando, Florida 32816, USA.
Department of Physics, Indiana State University, Terre Haute, Indiana 47809, USA.
Phys Rev Lett. 2020 Jul 3;125(1):017202. doi: 10.1103/PhysRevLett.125.017202.
Experimental observations of the ultrafast (less than 50 fs) demagnetization of Ni have so far defied theoretical explanations particularly since its spin-flipping time is much less than that resulting from spin-orbit and electron-lattice interactions. Through the application of an approach that benefits from spin-flip time-dependent density-functional theory and dynamical mean-field theory, we show that proper inclusion of electron correlations and memory (time dependence of electron-electron interaction) effects leads to demagnetization at the femtosecond scale, in good agreement with experimental observations. Furthermore, our calculations reveal that this ultrafast demagnetization results mainly from spin-flip transitions from occupied to unoccupied orbitals implying a dynamical reduction of exchange splitting. These conclusions are found to be valid for a wide range of laser pulse amplitudes. They also pave the way for ab initio investigations of ultrafast charge and spin dynamics in a variety of quantum materials in which electron correlations may play a definitive role.
到目前为止，镍的超快（小于50飞秒）退磁的实验观测结果一直无法得到理论解释，特别是因为其自旋翻转时间远小于自旋轨道和电子-晶格相互作用所导致的时间。通过应用一种受益于自旋翻转时间相关密度泛函理论和动态平均场理论的方法，我们表明，适当考虑电子关联和记忆（电子-电子相互作用的时间依赖性）效应会导致在飞秒尺度上的退磁，这与实验观测结果高度吻合。此外，我们的计算表明，这种超快退磁主要源于从占据轨道到未占据轨道的自旋翻转跃迁，这意味着交换分裂的动态减小。这些结论在很宽的激光脉冲振幅范围内都是有效的。它们还为对各种量子材料中的超快电荷和自旋动力学进行从头算研究铺平了道路，在这些材料中电子关联可能起着决定性作用。
